Microbumps are small diameter solder connections between a first die and a second die or between a die and a packaging substrate. The small diameter of the microbumps allows high density connections to the die, however, the high density may result in shorting or bridging between connections.
FIG. 1 is a cross-sectional view of two substrates connected through conventional microbumps. A first substrate 110 having copper pillars 112 faces a second substrate 120 having copper pillars 122. A solder 130 connects pillars of the copper pillars 112 with pillars of the copper pillars 122. The combination of the copper pillars 122 and the solder 130 is a microbump. When the pitch between the copper pillars 112, 122 is too small, bridging or shorting may occur such as, for example, in a region 132.
The small diameter of microbumps also increases the current density through the microbumps. Increases in current density cause electromigration in the microbumps. Electromigration is the movement of metal atoms resulting from momentum transfer by electrons to the metal atoms. Electromigration causes voids in the microbumps, which reduces reliability of the connections and leads to failure of integrated circuits containing the microbumps.
Microbumps are conventionally made from solder materials such as tin and silver, which suffer from electromigration. Copper reduces electromigration effects, but is too rigid for reliable assembly or operation in integrated circuits.
Thus, there is a need for a microbump structure with improved electromigration performance.